Selective Catalytic Reduction (SCR) systems normally receive as input a gas, and catalytically convert undesirable components in that gas into less noxious components. An example of such an SCR system is a catalytic reduction system in a diesel engine powered vehicle. One aspect of an SCR system is the conversion of nitrogen oxide (NO) and nitrogen dioxide (NO2) into nitrogen and water. In such SCR systems, a urea solution (which converts to ammonia—NH3) or other ammonia source is used to react with the nitrogen oxide and nitrogen dioxide. It is known that dosing the urea so that the ammonia is in a 1:1 ratio with the combined amount of both the nitrogen oxide and the nitrogen dioxide (referred to in the art as NOx) is conducive to the effectiveness of the system.